1. Field of Invention
This invention relates to a device that provides for a constant chlorine content in water. In particular, a simple, easily manipulated sensing electrode is provided that enables continuous monitoring of the free chlorine content of water. Electronic circuitry is provided for regulating chlorine additives to provide a desired content. Further, the device exhibits a very low transverse sensitivity to other materials which may be contained in water.
2. Description of the Prior Art
Because water used for drinking and bathing is primarily sterilized by chlorine, several methods have been developed to measure the free active chlorine content in aqueous solutions. The most widely used measurement means is the DPD method. Here, a chemical (diethyl-p-phenyldiamine) is added to a water sample contained in a standard measuring vessel and the sample color is compared to a color scale graduated in concentration steps.
The above technique can be automated. One device takes in water samples by means of a pump system. These samples are divided into two partial streams by means of an overflowing vessel, a so-called flow-through cuvette. One partial stream is mixed with a small, exactly defined quantity of a reagent and is transferred to the analysis cuvette. The second, untreated water sample is fed into a reference cuvette parallel to the treated sample cuvette. A zero-point adjustment is performed continuously and automatically. The reddish color of the treated sample water is measured and compared with the untreated water sample by a double beam photometer, and chlorine addition is adjusted accordingly.
Another prior art device uses the continuous through-flow analysis principle. Here a constant stream of sampled water flows out of a flow-through apparatus included in a mixer unit. Two impeller type pumps meter a buffered solution and a DPD-sulfate solution. When free chlorine is present, the concentration is indicated by a proportional amount of dye that is evaluated in a photometer attached to the measuring apparatus. A maximum of one minute response time elapses between the taking of the sample and the indication of the evaluated result. The measured value is read by a calibrated meter and additionally the measured value output provides a signal for continuous recording by a chart recorder.
With both of the above devices, the measured value at a given time can be converted to an electrical value and, by comparison with a reference value, regulation can be accomplished.
Electronic measuring procedures are also known. In a known arrangement, water flows past two electrodes made of two different metals and with these electrodes the water forms a voltaic cell. So long as the water contains no free active chlorine, the voltaic cell is substantially polarized. It delivers only a very low electrical current, termed residual current, for which electrical compensation is made. When free active chlorine is present, the polarization is disturbed. The voltaic cell becomes depolarized and delivers an electrical current whose magnitude is proportional to the concentration of free active chlorine. The electrode materials in the prior measurement cell are platinum and copper. Because the copper surface is subject to oxidation, quartz sand is placed within the measurement cell to act as a scouring agent. Due to the water circulation, the sand is kept in agitation and scours the electrodes so the copper electrode always presents a chemically clean surface. The device is calibrated after running idle for several hours by comparison with a DPD-measurement. The zero-point and measurement range adjustments must be redone approximately once a week as a routine scheduled monitoring of performance.
Another electronic method exists that operates according to the "Diffusion Limit-Current Method". Here a determined value of a unipotential voltage is applied to the operating electrode that is at a potential somewhat above the reduction potential of the ions to be displaced (in this case ClO.sup.- ions). At the electrode, stray ClO.sup.- ions are discharged. This results in a flow of electrical current. The stray ions following and coming from the adjacent medium maintain a flow of current, also termed "diffusion current", that is proportional to the concentration of free chlorine in the system. This measurement procedure is appropriate for stationary sample solutions. As a consequence of the requirements, the chlorine measurements in swimming pools and in potable water must have a continuous through-flow of the water being measured; the apparatus must have a device (i.e. pump) that produces an extremely constant flow. This method depends on the extreme accuracy of the polarization voltage. The polarization voltage and the electrode material determine the selectivity and, therefore, the accuracy of the device. Also, this method demands extremely expensive apparatus and is difficult to manipulate and handle.
The present invention relates to a metallic alloy, chlorine sensing electrode. Many patents have been issued concerning metal alloys used in electrodes. U.S. Pat. No. 4,127,468 discloses a metal electrode of a basis metal which is present in a finely divided or porous state and an alloying element. For the basis metal, any of the metals or their alloys from the Groups VIII, IB and IIB of the Periodic Table of Elements is used. The alloying-element is one selected from the groups IIIA IVA, VA, VIA, VIII, IB, IIB, or VIIIB of the Periodic Table of Elements.
U.S. Pat. No. 4,005,004 discloses an electrode used as an anode for electrolysis of an aqueous sodium chloride solution comprising an anti-corrosive conductor having a coating of a solid solution containing at least one noble metal oxide together with titanium oxide and zirconium oxide.